1. Field of the Invention
This invention relates to a method and apparatus for measuring a concentration of glucose in a living body. This invention particularly relates to a method and apparatus for measuring non-invasively the concentration of glucose in the aqueous humor in the anterior aqueous chamber of the eyeball, and a method and apparatus for measuring non-invasively the concentration of glucose in the blood in accordance with the concentration of glucose in the aqueous humor.
2. Description of the Prior Art
The mean level of glucose in the blood varies for different persons and is an important index for determining whether drugs are to be or are not to be administered to diabetic patients.
The concentration of glucose in the blood has the characteristics such that it fluctuates markedly within a very short time in accordance with food intake, physical activity, a complication by another disease, or the like. Urgent dosage is often required due to a sharp increase in concentration of blood glucose.
Therefore, as for patients having such a disease, it is desired that the concentration of glucose in the blood can be monitored at as short intervals as possible. Ordinarily, monitoring of the concentration of glucose in the blood is carried out by lancing the finger of the patient in order to obtain a drop of blood, analyzing the drop of blood, and thereby measuring the concentration of glucose in the blood. Since the lancing of the finger is painful, it is difficult to compel the patients to undergo the measurement procedure many times per day.
Accordingly, recently, in lieu of the invasive measurements having the drawbacks described above, various non-invasive measuring methods, which are not accompanied by pain, have been proposed.
The non-invasive measuring methods are primarily based upon the findings in that the concentration of glucose in the aqueous humor, which fills the anterior aqueous chamber located between the cornea and the crystalline lens of the human eyeball, has strong correlation with the concentration of glucose in the blood, though the level of correlation varies for different persons. With the non-invasive measuring methods, the concentration of glucose in the aqueous humor is measured non-invasively.
For example, a glucose sensor system, wherein the angle of rotation of infrared radiation having impinged upon the aqueous humor is measured, and the concentration of glucose having relationship with the angle of rotation is thereby determined, is proposed in, for example, U.S. Pat. No. 3,958,560.
Also, a technique for measuring stimulated Raman light from glucose is disclosed in, for example, WO 92/10131.
Further, a device for measuring the optical properties of light reflected from the crystalline lens of the eye is described in, for example, U.S. Pat. No. 5,535,743. Furthermore, a method for measuring the concentration of glucose in the aqueous humor is described in, for example, U.S. Pat. No. 5,433,197.
However, with the device described in U.S. Pat. No. 5,535,743, light reflected from the interface between the cornea and the aqueous humor cannot be eliminated, and information representing absorption at the cornea is detected together with the necessary information. Therefore, the accuracy, with which the concentration of glucose in the aqueous humor is determined, cannot be kept high.
Further, in U.S. Pat. No. 5,535,743, nothing is disclosed as to technical means to be used for measuring a minute change in absorbance. Therefore, the device described in U.S. Pat. No. 5,535,743 cannot be appropriately used in practice.
As for the glucose sensor system proposed in U.S. Pat. No. 3,958,560, many compounds other than glucose in the aqueous humor are optically active and take part in rotation of the plane of polarization. Also, the cornea exhibits birefringence and therefore causes rotation of the plane of polarization to occur. Accordingly, with the glucose sensor system proposed in U.S. Pat. No. 3,958,560, wherein the concentration of glucose in the aqueous humor is determined from the angle of rotation, the measurement accuracy cannot be kept high.
With the technique disclosed in WO 92/10131, in order for stimulated Raman light from glucose to be measured, a pump laser beam having a high intensity is introduced into the anterior aqueous chamber and in a direction normal to the vision line optical axis. Therefore, a practical measuring system cannot be constituted easily.